Step 1: Write the structure of serine. Serine is an amino acid with the structure: HOCH2-CH(NH2)-COOH. It belongs to the polar, neutral amino acid category. Step 2: Identify the functional groups present in serine. From the structure HOCH2-CH(NH2)-COOH, the functional groups are: (1) -COOH (carboxyl group), (2) -NH2 (amino group), and (3) -OH (hydroxyl group) on the side chain (R group). Step 3: Identify which groups are mentioned in the question. The question states that serine has -NH2 and -COOH groups. We need to find the ADDITIONAL functional group. Step 4: Identify the additional group. The third functional group in serine, besides -NH2 and -COOH, is the -OH (hydroxyl group) on the CH2 side chain. This -OH is an alcoholic hydroxyl group. Step 5: Explain why -OH makes serine special. The -OH group in serine's side chain can form hydrogen bonds and can be phosphorylated (phosphoserine), which is important in signal transduction. Serine is one of the three hydroxyl-containing amino acids (along with threonine and tyrosine). Step 6: Conclusion. The additional functional group in serine besides -NH2 and -COOH is the hydroxyl group (-OH). \[ \boxed{-OH \text{ (hydroxyl group, option 1)}} \]
